Method and apparatus for reconstructing an image block

ABSTRACT

In one embodiment, a motion vector for a current block is generated based on motion vector information for a block in a second picture layer and motion vector difference information associated with the current block. The second picture layer has lower quality pictures than pictures in the first picture layer, and the block of the second picture layer being temporally associated with the current block in the first picture layer. The current block is reconstructed using the generated motion vector and a reference picture.

DOMESTIC PRIORITY INFORMATION

This application claims the benefit of priority on U.S. ProvisionalApplication No. 60/723,474 filed Oct. 5, 2005; the entire content ofwhich is hereby incorporated by reference.

FOREIGN PRIORITY INFORMATION

This application claims the benefit of priority on Korean PatentApplication No. 10-2006-0068314 filed Jul. 21, 2006; the entire contentof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to methods of encoding anddecoding video signals.

2. Description of the Related Art

A Scalable Video Codec (SVC) is a scheme for encoding video signals atthe highest image quality when encoding the video signals, and enablingimage quality to be secured to some degree even if only part of theentire picture (frame) sequence generated as a result of the encoding (asequence of frames intermittently selected from the entire sequence) isdecoded.

Even if only a partial sequence of a picture sequence encoded by ascalable scheme is received and processed, image quality can be securedto some degree. However, if the bit rate is decreased, the deteriorationin image quality becomes serious. In order to solve the problem, aseparate sub-picture sequence for a low bit rate, for example, a picturesequence of small screens and/or a picture sequence having a smallnumber of frames per second, can be provided.

This sub-picture sequence is called a base layer, and a main picturesequence is called an enhanced layer. The base layer and the enhancedlayer are obtained by encoding the same video signal source, andredundant information exists in the video signals of the two layers.Therefore, when the base layer is provided, an interlayer predictionmethod can be used to improve coding efficiency.

Further, in order to improve the Signal-to-Noise Ratio (SNR) of a baselayer, that is, to enhance image quality, an enhanced layer may be used,which is called SNR scalability, Fine Granular Scalability (FGS), orprogressive refinement.

According to FGS, transform coefficients corresponding to respectivepixels, for example, Discrete Cosine Transform (DCT) coefficients, areseparately encoded into a base layer and an enhanced layer, depending onthe resolution of bit representation. When a transmission environment isbad, the transmission of the enhanced layer is omitted, so that the bitrate can be decreased while the quality of a decoded image isdeteriorated. That is, FGS compensates for loss occurring during aquantization process, and provides high flexibility enabling a bit rateto be controlled in response to a transmission or decoding environment.

For example, if a transform coefficient is quantized using aquantization step size (that is, QP), for example, QP=32, to generate abase layer, a first FGS enhanced layer is generated by quantizing thedifference between an original transform coefficient and a transformcoefficient obtained by inversely quantizing the quantized coefficientof the base layer, using a quantization step size corresponding toquality higher than QP=32, for example, QP=26. Similarly, a second FGSenhanced layer is generated by quantizing the difference between theoriginal transform coefficient and a transform coefficient obtained byinversely quantizing the sum of the quantized coefficients of the baselayer and the first FGS enhanced layer, using a quantization step size,for example, QP=20.

However, in a conventional FGS coding method, only a quality base layer,that is, a picture of an FGS base layer, is used to generate an FGSenhanced layer. This means that temporally redundant informationexisting between temporally adjacent quality enhanced layers, that is,pictures of an FGS enhanced layer, are not used.

In order to use such temporal redundancy in the FGS enhanced layer, amethod of utilizing an adjacent quality enhanced layer as well as aquality base layer to predict a current FGS enhanced layer is proposed.This method is called a Progressive FGS (PFGS), and the structure ofsuch a PFGS scheme is shown in FIG. 1.

As shown in FIG. 1, an adaptive reference block formation functionreceives a base layer collocated block Xb and a FGS enhanced layerreference block Re, and produces an adapted reference block Ra for usein reconstructing a current image block X in a current frame of the FGSlayer that is being reconstructed. The collocated block Xb is the blockin the base layer that is collocated with respect to the current imageblock X. Namely, the collocated block Xb is in a base layer frametemporally coincident with the current frame of the FGS layer, and thecollocated block Xb is in the same relative position within the baselayer frame as the current image block X in the current frame of the FGSlayer.

The collocated block Xb includes a reference picture index thatindicates a reference base layer frame. The collocated block Xb alsoincludes a motion vector. As shown in FIG. 1, the motion vector pointsto a base layer reference block Rb in the reference base layer frame.The FGS enhanced layer reference block Re is a collocated block withrespect to the base layer reference block Rb. Namely, the frame in theFGS layer temporally coincident with the reference frame in the baselayer indicated by the reference picture index of the collocated blockXb serves as the FGS enhanced layer reference frame. Further, the motionvector of the collocated block Xb is used as the motion vector in theFGS enhanced layer reference frame to obtain the FGS enhanced layerreference block Re.

The FGS enhanced layer reference block Re is a difference or errorsignal representing enhancement quality. As such, the adaptive referenceblock formation function adds the FGS enhanced layer reference block Reto the collocated block Xb at a transform coefficient level to obtainthe adapted reference block Ra. Then, as shown in FIG. 1, areconstruction function reconstructs the current image block X bycombining an encoded block Rd for the current image block X with theadapted reference block Ra in the well-known manner.

However, the resolution of bit representation of an image may vary dueto the difference between the quantization step sizes of the FGS baselayer and the FGS enhanced layer, so that the motion vector of the FGSbase layer collocated block Xb may not be identical to that of the FGSenhanced layer block X. This means that coding efficiency may bedecreased.

SUMMARY OF THE INVENTION

The present invention relates to a method of reconstructing a currentblock in a first picture layer.

In one embodiment, a motion vector for the current block is generatedbased on motion vector information for a block in a second picture layerand motion vector difference information associated with the currentblock. The second picture layer has lower quality pictures than picturesin the first picture layer, and the block of the second picture layerbeing temporally associated with the current block in the first picturelayer. The current block is reconstructed using the generated motionvector and a reference picture.

In one embodiment, a picture in the first picture layer is determined asthe reference picture based on a reference picture index for the blockin the second picture layer.

In one embodiment, the motion vector information is obtained from theblock in the second picture layer.

In one embodiment, the motion vector is generated by determining amotion vector prediction based on the obtained motion vectorinformation, and generating the motion vector associated with thecurrent block in the first picture layer based on the motion vectorprediction and the motion vector difference information.

For example, the motion vector information may include a motion vectorassociated with the block of the second picture layer, and the motionvector prediction may be determined equal to the motion vectorassociated with the block of the second picture layer.

In one embodiment, the reference picture for the current block may betemporally associated with a reference picture in the second picturelayer, and the reference picture in the second picture layer is areference picture for the block in the second picture layer.

The present invention also relates to an apparatus for reconstructing acurrent block in a first picture layer.

In one embodiment, the apparatus includes a first decoder generating amotion vector for the current block based on motion vector informationfor a block in a second picture layer and motion vector differenceinformation associated with the current block. The second picture layerhas lower quality pictures than pictures in the first picture layer, andthe block of the second picture layer is temporally associated with thecurrent block in the first picture layer. The first decoder reconstructsthe current block using the generated motion vector. The apparatus alsoincludes a second decoder obtaining the motion vector information fromthe second picture layer and sending the motion vector information tothe first decoder.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a progressive FGS structure for encoding the FGSenhanced layer of a current frame using the quality base layer of thecurrent frame and the quality enhanced layer of another frame;

FIG. 2 illustrates a process of finely adjusting the motion vector ofthe FGS base layer of a current frame in the picture of the FGS enhancedlayer of a reference frame to predict the FGS enhanced layer of thecurrent frame according to an embodiment of the present invention;

FIG. 3 illustrates a process of searching the FGS enhanced layer pictureof a reference frame for an FGS enhanced layer reference block for anarbitrary block in a current frame, independent of the motion vector ofan FGS base layer of the arbitrary block according to another embodimentof the present invention;

FIG. 4 is a block diagram of an apparatus which encodes a video signalto which the present invention may be applied; and

FIG. 5 is a block diagram of an apparatus which decodes an encoded datastream to which the present invention may be applied.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Hereinafter, example embodiments of the present invention will bedescribed in detail with reference to the attached drawings.

In an embodiment of the present invention, during the encoding process,the motion vector mv(Xb) of a Fine Granular Scalability (FGS) base layercollocated block Xb is finely adjusted to improve the coding efficiencyof Progressive FGS (PFGS).

That is, the embodiment obtains the FGS enhanced layer frame for the FGSenhanced layer block X to be encoded as the FGS enhanced layer frametemporally coincident with the base layer reference frame for the baselayer block Xb collocated with respect to the FGS enhanced layer blockX. As will be appreciated, this base layer reference frame will beindicated in a reference picture index of the collocated block Xb;however, it is common for those skilled in the art to refer to thereference frame as being pointed to by the motion vector. Given theenhanced layer reference frame, a region (e.g., a partial region) of apicture is reconstructed from the FGS enhanced layer reference frame.This region includes a block indicated by the motion vector mv(Xb) forthe base layer collated block Xb. The region is searched to obtain theblock having the smallest image difference with respect to the block X,that is, a block Re′, causing the Sum of Absolute Differences (SAD) tobe minimized. The SAD is the sum of absolute differences betweencorresponding pixels in the two blocks. The two blocks are the block Xto be coded or decoded and the selected block. Then, a motion vectormv(X) from the block X to the selected block is calculated.

In this case, in order to reduce the burden of the search, the searchrange can be limited to a region including predetermined pixels inhorizontal and vertical directions around the block indicated by themotion vector mv(Xb). For example, the search can be performed withrespect only to the region extended by 1 pixel in every direction.

Further, the search resolution, that is, the unit by which the block Xis moved to find a block having a minimum SAD, may be a pixel, a ½ pixel(half pel), or a ¼ pixel (quarter pel).

In particular, when a search is performed with respect only to theregion extended by 1 pixel in every direction, and is performed on apixel basis, the location at which SAD is minimized is selected fromamong 9 candidate locations, as shown in FIG. 2.

If the search range is limited in this way, the difference vectormvd_ref fgs between the calculated motion vector mv(X) and the motionvector mv(Xb), as shown in FIG. 2, is transmitted in the FGS enhancedlayer. The FGS enhanced layer reference block associated with theobtained motion vector mv(x) is the enhanced layer reference block Re′.The block Re is used as a prediction block (or a predictor) for theblock X to be decoded.

In another embodiment of the present invention, in order to obtain anoptimal motion vector mv_fgs for the FGS enhanced layer for the block X,that is, in order to generate the optimal predicted image of the FGSenhanced layer for the block X, motion estimation/prediction operationsare performed independent of the motion vector mv(Xb) for the FGS baselayer collocated block Xb corresponding to the block X, as shown in FIG.3.

In this case, the FGS enhanced layer predicted image (FGS enhanced layerreference block) for the block X can be searched for in the referenceframe indicated by the motion vector mv(Xb) (i.e., indicated by thereference picture index for the block Xb), or the reference block forthe block X can be searched for in another frame. As with the embodimentof FIG. 2, the obtained FGS enhanced layer reference block associatedwith the motion vector mv(X) is the enhanced layer reference block Re′.

In the former case, there are advantages in that frames in which the FGSenhanced layer reference block for the block X is to be searched for arelimited to the reference frame indicated by the motion vector mv(Xb), sothat the burden of encoding is reduced, and there is no need to transmita reference index for the block X that includes the reference block.

In the latter case, there are disadvantages in that the number offrames, in which the reference block is to be searched for, increases,so that the burden of encoding increases, and a reference index for theframe, including a found reference block, must be additionallytransmitted. But, there is an advantage in that the optimal predictedimage of the FGS enhanced layer for the block X can be generated.

When a motion vector is encoded without change, a great number of bitsare required. Since the motion vectors of neighboring blocks have atendency to be highly correlated, respective motion vectors can bepredicted from the motion vectors of surrounding blocks that have beenpreviously encoded (immediate left, immediate upper and immediateupper-right blocks).

When a current motion vector mv is encoded, generally, the differencemvd between the current motion vector mv and a motion vector mvp, whichis predicted from the motion vectors of surrounding blocks, is encodedand transmitted.

Therefore, the motion vector mv_fgs of the FGS enhanced layer for theblock X that is obtained through an independent motion predictionoperation is encoded by mvd_fgs=mv_fgs−mvp_fgs. In this case, the motionvector mvp_fgs, predicted and obtained from the surrounding blocks, canbe implemented using the motion vector mvp, obtained when the motionvector mv(Xb) of the FGS base layer collocated block Xb is encoded,without change (e.g., mvp=mv(Xb)), or using a motion vector derived fromthe motion vector mvp (e.g., mvp=scaled version of mv(Xb)).

If the number of motion vectors of the FGS base layer collocated blockXb corresponding to the block X is two, that is, if the block Xb ispredicted using two reference frames, two pieces of data related to theencoding of the motion vector of the FGS enhanced layer for the block Xare obtained. For example, in a first embodiment, the pieces of data aremvd_ref_fgs_10/11, and in a second embodiment, the pieces of data aremvd_fgs_10/11.

In the above embodiments, the motion vectors for macroblocks (or imageblocks smaller than macroblocks) are calculated in relation to the FGSenhanced layer, and the calculated motion vectors are included in amacroblock layer within the FGS enhanced layer and transmitted to adecoder. However, in the conventional FGS enhanced layer, relatedinformation is defined on the basis of a slice level, and is not definedon the basis of a macroblock level, a sub-macroblock level, or sub-blocklevel.

Therefore, in the present invention, in order to define, in the FGSenhanced layer, data related to the motion vectors calculated on thebasis of a macroblock (or an image block smaller than a macroblock),syntax required to define a macroblock layer and/or an image block layersmaller than a macroblock layer, for example,progressive_refinement_macroblock_layer_in_scalable_extension( ), andprogressive_refinement_mb (and/or sub_mb)_pred_in_scalable_extension( ),is newly defined, and the calculated motion vectors are recorded in thenewly defined syntax and then transmitted.

Meanwhile, the generation of the FGS enhanced layer is similar to aprocedure of performing prediction between a base layer and an enhancedlayer having different spatial resolutions in an intra base predictionmode, and generating residual data which is an image difference.

For example, if it is assumed that the block of the enhanced layer is Xand the block of the base layer corresponding to the block X is Xb, theresidual block obtained through intra base prediction is R=X−Xb. In thiscase, X can correspond to the block of a quality enhanced layer to beencoded, Xb can correspond to the block of a quality base layer, andR=X−Xb can correspond to residual data to be encoded in the FGS enhancedlayer for the block X.

In another embodiment of the present invention, an intra mode predictionmethod is applied to the residual block R to reduce the amount ofresidual data to be encoded in the FGS enhanced layer. In order toperform intra mode prediction on the residual block R, the same modeinformation about the intra mode that is used in the base layercollocated block Xb corresponding to the block X is used.

A block Rd having a difference value of the residual data is obtained byapplying the mode information, used in the block Xb, to the residualblock R. Discrete Cosine Transform (DCT) is performed on the obtainedblock Rd, and the DCT results are quantized using a quantization stepsize set smaller than the quantization step size used when the FGS baselayer data for the block Xb is generated, thus generating FGS enhancedlayer data for the block X.

In a further embodiment, an adapted reference block Ra′ for the block Xis generated as equal to the FGS enhanced layer reference block Re′.Further, residual data R to be encoded in the FGS enhanced layer for theblock X is set as R=X−Ra, so that an intra mode prediction method isapplied to the residual block R. It will be appreciated that in thisembodiment, the enhanced layer reference block Re′, and therefore, theadapted reference block Ra′, are reconstructed pictures and not at thetransform coefficient level.

In this case, an intra mode applied to the residual block R is a DC modebased on the mean value of respective pixels in the block R. Further, ifthe block Re is generated by the methods according to embodiments of thepresent invention, information related to motion required to generatethe block Re in the decoder must be included in the FGS enhanced layerdata for the block X.

FIG. 4 is a block diagram of an apparatus which encodes a video signaland to which the present invention may be applied.

The video signal encoding apparatus of FIG. 4 includes a base layer (BL)encoder 110 for performing motion prediction on an image signal, inputas a frame sequence, using a predetermined method; performing DCT onmotion prediction results; quantizing the DCT transform results, using apredetermined quantization step size; and generating base layer data. AnFGS enhanced layer (FGS_EL) encoder 120 generates the FGS enhanced layerof a current frame using the motion information, the base layer datathat are provided by the BL encoder 110, and the FGS enhanced layer dataof a frame (for example, a previous frame) which is a reference formotion estimation for the current frame. A muxer 130 multiplexes theoutput data of the BL encoder 110 and the output data of the FGS_ELencoder 120 using a predetermined method, and outputs multiplexed data.

The FGS_EL encoder 120 reconstructs the quality base layer of thereference frame (also called a FGS base layer picture), which is thereference for motion prediction for a current frame, from the base layerdata provided by the BL encoder 110, and reconstructs the FGS enhancedlayer picture of the reference frame using the FGS enhanced layer dataof the reference frame and the reconstructed quality base layer of thereference frame.

In this case, the reference frame may be a frame indicated by the motionvector mv(Xb) of the FGS base layer collocated block Xb corresponding tothe block X in the current frame.

When the reference frame is a frame previous to the current frame, theFGS enhanced layer picture of the reference frame may have been storedin a buffer in advance.

Thereafter, the FGS_EL encoder 120 searches the FGS enhanced layerpicture of the reconstructed reference frame for an FGS enhanced layerreference image for the block X, that is, a reference block or predictedblock Re′ in which an SAD with respect to the block X is minimized, andthen calculates a motion vector mv(X) from the block X to the foundreference block Re′.

The FGS_EL encoder 120 performs DCT on the difference between the blockX and the found reference block Re′, and quantizes the DCT results usinga quantization step size set smaller than a predetermined quantizationstep (quantization step size used when the BL encoder 110 generates theFGS base layer data for the block Xb), thus generating FGS enhancedlayer data for the block X.

When the reference block is predicted, the FGS_EL encoder 120 may limitthe search range to a region including predetermined pixels inhorizontal and vertical directions around the block indicated by themotion vector mv(Xb) so as to reduce the burden of the search, as in thefirst embodiment of the present invention. In this case, the FGS_ELencoder 120 records the difference mvd_ref_fgs between the calculatedmotion vector mv(X) and the motion vector mv(Xb) in the FGS enhancedlayer in association with the block X.

Further, as in the case of the above-described second embodiment of thepresent invention, the FGS_EL encoder 120 may perform a motionestimation operation independent of the motion vector mv(Xb) so as toobtain the optimal motion vector mv_fgs of the FGS enhanced layer forthe block X; thus searching for a reference block Re′ having a minimumSAD with respect to the block X, and calculating the motion vectormv_fgs from the block X to the found reference block Re.

In this case, the FGS enhanced layer reference block for the block X maybe searched for in the reference frame indicated by the motion vectormv(Xb), or a reference block for the block X may be searched for in aframe other than the reference frame.

The FGS_EL encoder 120 performs DCT on the difference between the blockX and the found reference block Re′, and quantizes the DCT results usinga quantization step size set smaller than the predetermined quantizationstep size; thus generating the FGS enhanced layer data for the block X.

Further, the FGS_EL encoder 120 records the difference mvd_fgs betweenthe calculated motion vector mv_fgs and the motion vector mvp_fgs,predicted and obtained from surrounding blocks, in the FGS enhancedlayer in association with the block X. That is, the FGS_EL encoder 120records syntax for defining information related to the motion vectorcalculated on a block basis (a macroblock or an image block smaller thana macroblock), in the FGS enhanced layer.

When the reference block Re′ for the block X is searched for in a frameother than the reference frame indicated by the motion vector mv(Xb),information related to the motion vector may further include a referenceindex for a frame including the found reference block Re′.

The encoded data stream is transmitted to a decoding apparatus in awired or wireless manner, or is transferred through a recording medium.

FIG. 5 is a block diagram of an apparatus which decodes an encoded datastream and to which the present invention may be applied. The decodingapparatus of FIG. 5 includes a demuxer 210 for separating a receiveddata stream into a base layer stream and an enhanced layer stream; abase layer (BL) decoder 220 for decoding an input base layer streamusing a preset method; and an FGS_EL decoder 230 for generating the FGSenhanced layer picture of a current frame using the motion information,the reconstructed quality base layer (or FGS base layer data) that areprovided by the BL decoder 220, and the FGS enhanced layer stream.

The FGS_EL decoder 230 checks information about the block X in thecurrent frame, that is, information related to a motion vector used formotion prediction for the block X, in the FGS enhanced layer stream.

When i) the FGS enhanced layer for the block X in the current frame isencoded on the basis of the FGS enhanced layer picture of another frameand ii) is encoded using a block other than the block indicated by themotion vector mv(Xb) of the block Xb corresponding to the block X (thatis the FGS base layer block of the current frame) as a predicted blockor a reference block, motion information for indicating the other blockis included in the FGS enhanced layer data of the current frame.

That is, in the above description, the FGS enhanced layer includessyntax for defining information related to the motion vector calculatedon a block basis (a macroblock or an image block smaller than amacroblock). The information related to the motion vector may furtherinclude an index for the reference frame in which the FGS enhanced layerreference block for the block X is found (the reference frame includingthe reference block).

When motion information related to the block X in the current frameexists in the FGS enhanced layer of the current frame, the FGS_ELdecoder 230 generates the FGS enhanced layer picture of the referenceframe using the quality base layer of the reference frame (the FGS baselayer picture reconstructed by the BL decoder 220 may be provided, ormay be reconstructed from the FGS base layer data provided by the BLdecoder 220), which is the reference for motion prediction for thecurrent frame, and the FGS enhanced layer data of the reference frame.In this case, the reference frame may be a frame indicated by the motionvector mv(Xb) of the block Xb.

Further, the FGS enhanced layer of the reference frame may be encodedusing an FGS enhanced layer picture of a different frame. In this case,a picture reconstructed from the different frame is used to reconstructthe reference frame. Further, when the reference frame is a frameprevious to the current frame, the FGS enhanced layer picture may havebeen generated in advance and stored in a buffer.

Further, the FGS_EL decoder 230 obtains the FGS enhanced layer referenceblock Re′ for the block X from the FGS enhanced layer picture of thereference frame, using the motion information related to the block X.

In the above-described first embodiment of the present invention, themotion vector mv(X) from the block X to the reference block Re′ isobtained as the sum of the motion information mv_ref_fgs, included in anFGS enhanced layer stream for the block X, and the motion vector mv(Xb)of the block Xb.

Further, in the second embodiment of the present invention, the motionvector mv(X) is obtained as the sum of the motion information mvd_fgs,included in the FGS enhanced layer stream for the block X, and themotion vector mvp_fgs, predicted and obtained from the surroundingblocks. In this case, the motion vector mvp_fgs may be implemented usingthe motion vector mvp, which is obtained at the time of calculating themotion vector mv(Xb) of the FGS base layer collocated block Xb withoutchange, or using a motion vector derived from the motion vector mvp.

Thereafter, the FGS_EL decoder 230 performs inverse-quantization andinverse DCT on the FGS enhanced layer data for the block X, and adds theresults of inverse quantization and inverse DCT to the obtainedreference block Re′, thus generating the FGS enhanced layer picture forthe block X.

The above-described decoding apparatus may be mounted in a mobilecommunication terminal, or a device for reproducing recording media.

As described above, the present invention is advantageous in that it canefficiently perform motion estimation/prediction operations on an FGSenhanced layer picture when the FGS enhanced layer is encoded ordecoded, and can efficiently transmit motion information required toreconstruct an FGS enhanced layer picture.

Although the example embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the invention.

1. A method of reconstructing a current block in a first picture layer,comprising: generating a motion vector for the current block based onmotion vector information for a block in a second picture layer andmotion vector difference information associated with the current block,the second picture layer having lower quality pictures than pictures inthe first picture layer, and the block of the second picture layer beingtemporally associated with the current block in the first picture layer;and reconstructing the current block using the generated motion vectorand a reference picture.
 2. The method of claim 1, further comprising:determining the reference picture, which is in the first picture layer,based on a reference picture index for the block in the second picturelayer.
 3. The method of claim 1, further comprising: obtaining themotion vector information from the block in the second picture layer;and obtaining the motion vector difference information from a video databitstream.
 4. The method of claim 1, wherein the motion vectorinformation includes a motion vector associated with the block of thesecond picture layer.
 5. The method of claim 1, wherein the generatingstep comprises: determining a motion vector prediction based on theobtained motion vector information; and generating the motion vectorassociated with the current block in the first picture layer based onthe motion vector prediction and the motion vector differenceinformation.
 6. The method of claim 5, wherein the motion vectorinformation includes a motion vector associated with the block of thesecond picture layer; and the determining a motion vector predictionstep determines the motion vector prediction equal to the motion vectorassociated with the block of the second picture layer.
 7. The method ofclaim 5, wherein the generating step generates the motion vector for thecurrent block equal to the motion vector prediction plus a motion vectordifference indicated by the motion vector difference information.
 8. Themethod of claim 7, wherein the motion vector information includes amotion vector associated with the block of the second picture layer; andthe determining a motion vector prediction step determines the motionvector prediction equal to the motion vector associated with the blockof the second picture layer.
 9. The method of claim 1, wherein themotion vector difference information indicates a motion vectordifference of a one-quarter pixel or less.
 10. The method of claim 1,wherein the motion vector difference information indicates a motionvector difference of a one-half pixel or less.
 11. The method of claim1, wherein the reference picture is a picture in the first picturelayer.
 12. The method of claim 11, wherein the reference picture for thecurrent block is temporally associated with a reference picture in thesecond picture layer, the reference picture in the second picture layerbeing a reference picture for the block in the second picture layer. 13.The method of claim 1, wherein the reconstructing step combines aprediction block with a residual block to reconstruct the current block,the prediction block being based on the generated motion vector and thereference picture.
 14. An apparatus for reconstructing a current blockin a first picture layer, comprising: a first decoder generating amotion vector for the current block based on motion vector informationfor a block in a second picture layer and motion vector differenceinformation associated with the current block, the second picture layerhaving lower quality pictures than pictures in the first picture layer,and the block of the second picture layer being temporally associatedwith the current block in the first picture layer; the first decoderreconstructing the current block using the generated motion vector; anda second decoder obtaining the motion vector information from the secondpicture layer and sending the motion vector information to the firstdecoder.